This invention relates in general to a connector for maintaining a fixed spacing between two pieces of apparatus and relates more particularly to a connector that transmits only a negligible amount of vibration from a first of these two apparatus to a second of these two apparatus. Such a connection is useful in automated vibration sensitive tools, such as a registration tool, which is utilized to calibrate and correct a stepper that is to produce accurate alignment between successive layers of an integrated circuit.
Integrated circuits typically consist of several layers of material that are patterned and interconnected in such a manner that these layers produce the desired integrated circuit. There are many different processes for producing patterned layers, but regardless of the manner of producing each layer, in order to achieve an acceptable yield of good circuits, it is crucial to accurately align every patterned layer with all other patterned layers of that circuit.
In a typical wafer fabrication process, each of these patterned layers is formed by the steps of: (a) depositing on the wafer a layer of resist; (b) exposing this layer with radiation to produce a pattern of exposed regions in this layer; (c) developing the resist to produce a contact mask; and then (d) processing the wafer through this contact mask. In these steps, the resist can, for example, be a photoresist that is exposed by light imaged through a projection mask or can be a resist that is sensitive to incidence of an electron beam that is controlled to produce the desired pattern of exposure in the resist. In each of these cases, the resist layer is developed to produce the contact mask. The contact mask can be used, for example, during deposition or implant steps to determine where material is added to the wafer and can also be used, for example, during etching steps to determine where material is removed from the wafer.
In the future, it may be possible to produce patterned layers on the wafer by mechanisms that do not utilize either a projection mask or a contact mask. Therefore, the apparatus utilized to produce a patterned layer on the wafer will be referred to herein generically as the "pattern generator".
In most integrated circuit manufacturing processes, a stepper is utilized to produce concurrently a two dimensional pattern of identical integrated circuits on an integrated circuit wafer, thereby greatly increasing the throughput of the integrated circuit manufacturing process. This stepper can accurately translate an integrated circuit wafer in each of two perpendicular directions that are parallel to a major planar surface of this wafer.
To achieve a commercially acceptable yield of good integrated circuits, it is critical that all of the layers are accurately aligned translationally and rotationally. Because the patterns are typically produced on a wafer by projection techniques, it is also critical that the magnification of the projection be correct. Lens aberrations can also affect the pattern being projected. Therefore, once a pattern is ready to be projected onto the wafer, a test wafer is produced that is then measured by a registration tool to determine whether there is any translational or rotational misalignment and whether the magnification of the projection is correct. If one or more of these parameters is incorrect, the stepper is corrected and another test wafer is produced and tested. This is repeated until accurate alignment is achieved. FIG. 1 is a schematic view of the front panel of such a registration tool.
Within registration tool 11, a microscope magnifies the alignment marks sufficiently (typically, a magnification on the order of 1,000 times) that the desired alignment accuracy is achieved. A light source illuminates the wafer within a microscope field of view that has a diameter typically on the order of 250 microns. Within this field of view is a smaller region of diameter on the order of 50 microns within which there is negligible optical distortion. This latter region is referred to herein as the "distortionless region" of the microscope.
Several alignment marks are spaced throughout the test wafer so that the locations of several ma ks can be measured to determine whether the translational and rotational parameters are correct and whether the magnification is correct. Measurement of the locations of these alignment marks was historically achieved manually, but is preferably achieved automatically to reduce the time required to make such measurements and to improve the accuracy and repeatability of such measurements. Likewise, as illustrated in the block diagram of FIG. 2, for improved throughput, it is advantageous to utilize a robot 21 to transfer wafers between a wafer stack 22 and a microscope 23.
For production of 4 Megabit RAMS, the maximum allowable misalignment between the circuit being processed and the pattern generator is approximately 150 nanometers, thereby requiring that microscope 23 be substantially free of vibration during every measurement step. The allowable tolerance for a measurement tool is typically 10% of the allowable tolerance of the pattern generator. It is therefore important to prevent transmitting to microscope 23 any vibrations from such sources as robot 21, a keyboard 12 mounted on the outside enclosure of registration tool 11 and cooling fans within registration tool 11. Therefore, microscope 23 mounted on an isolation stand 24 that makes no direct physical contact with either robot 21 or with the outer enclosure 13 on such fans (not shown) are mounted to cool the inside of registration tool 11.
This isolation stand is much more massive than the robot and the rest of the system, so that any vibrational energy coupled from the robot or the environment to the isolation stand is distributed over this large mass, thereby reducing the magnitude of vibration of any portion of this isolation stand. The mass of the isolation stand is typically on the order of 2,000 pounds. Isolation stand 24 has a plurality of legs 16, each of which is supported on a pad 14 of damping material (such as Isodamp, available from E.A.R. Corp, located at 7911 Zionsville Road, lndianopolis, Ind. 46268-0898) that absorbs most of the vibrational energy coupled through the floor to the isolation stand. By "damping material" is meant any material such as these Isodamp pads that strongly absorb vibrational energy. This isolation enables microscope 21 to provide a crisp image of the wafer, thereby enabling accurate measurement of the translational and rotational alignment of each circuit with the pattern generator during processing of that circuit by the Pattern generator.
Critical alignment is essential between the robot and the microscope to allow the system to quickly find taught alignment targets. Unfortunately, if registration tool 11 is inadvertently bumped, this can disturb alignment between the microscope and the robot sufficiently that no alignment mark will be present within the field of view of the microscope. If such Misalignment between the robot and the microscope is small enough, execution of a spiral search pattern by the microscope stage can locate an alignment mark and then home in to that mark. Although occurrences of such small amounts of misalignment and the associated implementation of a search pattern only degrade throughput, larger amounts of misalignment between the robot and the microscope will cause the taught pattern to fall outside of the Spiral search area, causing an automated system to reject the wafer. Therefore, it would be advantageous to have a connector that maintains alignment between the robot and the microscope without coupling a significant amount of vibration to the microscope from the robot, keyboard, fans and other sources of vibration within the registration tool.